Recently, designers of portable and other low power electronic devices have devised multimedia features in order for their products to attract more attention from potential customers. Internally to these devices, subsystem circuits such as audio subsystems have an increasingly important role on the realisation of these features and, thus, on user experience.
A typical audio subsystem, for instance, may combine in a single integrated circuit, various audio configurations for driving multiple output speakers such as a headphone, a hands-free loudspeaker and a receiver speaker, each of these audio configurations potentially having respective output power requirements. For instance, an audio subsystem may use a Class-D amplifier in different audio configurations for driving a circuit such as a hands-free loudspeaker. In fact, depending on the user's need and due to its high power and high efficiency, a Class-D amplifier may provide different levels of loudness. For example, when a medium loudness is required, e.g. 1 W, the Class-D amplifier may be powered by the battery of the device. In this configuration the sound loudness would depend on the battery charge state. In another example, when a high loudness is required, e.g. 2 W, the Class-D amplifier may be powered by a DC-DC boost converter. In this configuration, the sound loudness could be constant whatever the battery charge state.
However, with such type of audio subsystems, when only the medium loudness audio configuration is used the DC-DC boost converter is not used. This is inefficient and represents extra subsystems costs due to the unused die area.